This application is based on Japanese Patent Application No. 2000-377367 filed on Dec. 12, 2000, the contents of which are incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to improvements of a shift control apparatus for controlling a continuously variable transmission device of an automotive vehicle, and more particularly to techniques for assuring a shift-down action of a continuously variable transmission mechanism of the transmission device toward its lowest-speed position having the highest speed reduction ratio, before the vehicle is re-started.
2. Discussion of Related Art
A continuously variable transmission device of an automotive vehicle is generally controlled so as to be shifted down toward its lowest-speed position having the highest speed reduction ratio, when the vehicle is stopped. Where the vehicle is abruptly stopped on an uphill road, for instance, the vehicle may be brought to a stop before completion of the intended shift-down action of a continuously variable transmission mechanism of the transmission device to establish the lowest-speed position. In the continuously variable transmission, it is difficult to change the speed reduction ratio after rotary motions of rotary elements of the continuously variable transmission mechanism have been stopped. If the transmission mechanism has not been placed in the lowest-speed position before re-starting of the vehicle, the vehicle cannot be re-started with a sufficient drive force. Thus, it is difficult to re-start the vehicle when the transmission mechanism is not placed in the lowest-speed position.
In view of the above drawback, there has been proposed a shift control apparatus arranged to effect a compulsory shift-down action of the continuously variable transmission mechanism to increase the speed reduction ratio toward the highest value upon re-starting the vehicle after stopping of the vehicle without the highest speed reduction ratio being established, even before initiation of the rotary motions of the rotary elements of the transmission mechanism, that is, even while the rotary elements of the transmission mechanism are at a standstill or at rest. This arrangement permits the vehicle to be re-started with a sufficient drive force. An example of such a transmission shift control apparatus is disclosed in JP-A-3-292452.
In the known shift control apparatus described above, however, the compulsory shift-down action of the continuously variable transmission toward its lowest-speed position upon re-starting of the vehicle during application of a drive power to the continuously variable transmission mechanism causes slipping on the friction surfaces of the rotary elements in the power transmitting direction, with a result of undesirable friction on the friction surfaces of the rotary elements.
The present invention was made in view of the background art discussed above. It is therefore an object of the present invention to provide a shift control apparatus for controlling a continuously variable transmission, which permits the transmission to be shifted down toward the lowest-speed position, while preventing or minimizing the friction on the friction surfaces of the rotary elements of the transmission.
The above object may be achieved according to the principle of this invention, which provides a shift control apparatus for controlling a continuously variable transmission device including a continuously variable transmission mechanism having rotary elements and operable to transmit a rotary motion of a drive power source of an automotive vehicle to a drive wheel of the automotive vehicle, such that a speed ratio of the mechanism is continuously variable, the mechanism including rotary elements, comprising: (a) power-transmission-state determining means for determining whether the continuously variable transmission mechanism is placed in a non-power-transmitting state in which the mechanism is not transmitting a drive force; (b) speed-ratio determining means for determining whether the continuously variable transmission mechanism is placed in a lowest-speed position; (c) rotation-stop detecting means for determining whether the rotary elements are at rest; and (d) compulsory-speed-ratio-change implementing means for effecting a compulsory shift-down action of the continuously variable transmission mechanism toward the lowest-speed position when the mechanism is placed in the non-power-transmitting state while the continuously variable transmission mechanism is not placed in the lowest-speed position and while the rotary elements of the mechanism are at rest.
In the shift-control apparatus of the present invention constructed as described above, the compulsory-speed-ratio-change implementing means is operated to effect the compulsory shift-down action of the continuously variable transmission mechanism, when the power-transmission-state determining means determines that the transmission mechanism is placed in the non-power-transmitting state while the speed-ratio determining means determines that the transmission mechanism is not placed in its lowest-speed position and while the rotation-stop detecting means determines that the variable-diameter pulleys are at a standstill or at rest. The compulsory shift-down action of the transmission mechanism is effected by the compulsory-speed-ratio-change implementing means, to establish the lowest-speed position prior to re-starting the vehicle. Further, the compulsory shift-down action of the transmission mechanism by the compulsory-speed-ratio-change implementing means to change the speed ratio toward the value of the lowest-speed position does not cause frictional sliding actions of a power transmitting member on the friction surfaces of the rotary elements in the power transmitting circumferential direction, since the compulsory shift-down action is effected while the transmission mechanism is placed in the non-power-transmitting state in which the transmission mechanism does not transmitting a drive force. In other words, the compulsory shift-down action causes only sliding actions of the power transmitting member in the radial direction, and the speed of these radial sliding actions is considerably low than that of the circumferential sliding actions, so that the amount of wear of the friction surfaces of the rotary elements is significantly reduced. The speed ratio may be defined as a speed reduction ratio, which is a ratio of an input speed of the transmission mechanism to an output speed of the transmission mechanism. In this case, the transmission mechanism has the highest speed reduction ration when it is placed in the lowest-speed position, and the compulsory shift-down action cause the speed reduction ratio to be increased to the highest value.
A first preferred form of the shift control apparatus of the invention is applicable to an automotive vehicle includes a manually operable member which is operable to select one of a plurality of operating states of the continuously variable transmission device and which has a drive position for selecting a power-transmitting state of the transmission device, and a non-drive position for selecting the non-power-transmitting state. In this form of the shift control apparatus, the power-transmission-state determining means is operable to determine that the continuously variable transmission mechanism the is placed in the non-power-transmitting state when a predetermined time has elapsed after the manually operable member has been operated from the drive position to the non-drive position. Since the power transmitting path of the continuously variable transmission device is disconnected the predetermined time after the manually operable member is operated to the non-drive position, the non-power-transmitting state of the transmission mechanism can be highly reliably detected by the power-transmission-state determining means.
According to a second preferred form of the shift control apparatus of this invention, the power-transmission-state determining means is operable to determine that the continuously variable transmission mechanism is placed in the non-power-transmitting state when an input torque of the transmission mechanism has been substantially zeroed. This arrangement permits accurate detection of the non-power-transmitting state of the transmission mechanism, even when substantially the entire output torque of the drive power source (e.g., internal combustion engine) is consumed by an optional device such as an electric generator (for charging a battery) or an air conditioner. The present arrangement permits the compulsory shift-down action of the transmission mechanism even when the drive power source is operated to operate such an optional device.
A third preferred form of the shift control apparatus further comprises time-elapse detecting means for determining whether a predetermined time duration has elapsed after a moment of initiation of the compulsory shift-down action of the continuously variable transmission mechanism by the compulsory-speed-ratio-change implementing means, and the compulsory-speed-ratio-change implementing means is arranged to terminate the compulsory shift-down action when the predetermined time duration has passed after the moment of initiation of the compulsory shift-down action. This arrangement is effective to prevent continuation of the compulsory shift-down action for an unnecessarily long time duration.
In one advantageous arrangement of the above-described third preferred form of the shift control apparatus, the predetermined time duration for which the compulsory shift-down action is effected is determined on the basis of a detected temperature of a working fluid of the continuously variable transmission device, and according to a predetermined relationship between the time duration and the temperature of the working fluid. This arrangement permits the shift-down action of the transmission mechanism for the required minimum time, irrespective of a variation in the viscosity of the working fluid with a change in the fluid temperature.
In another advantageous arrangement of the shift control apparatus, the predetermined time duration is determined on the basis of the speed ratio of the continuously variable transmission mechanism upon initiation of the compulsory shift-down action, and according to a predetermined relationship between the time duration and the speed ratio. This arrangement permits the shift-down action of the transmission mechanism for the required minimum time, which varies with the speed ratio upon initiation of the shift-down action of the transmission mechanism.